Torque wrenches are known hand tools which are used in manufacturing industry for example on assembly lines to tighten bolts and other threaded fasteners to a recommended minimum tightness. It is increasingly important in production line manufacture to control and monitor the maximum and minimum torque to which threaded fastener joints are tightened. The use of alloys of relatively soft and lightweight metals for components does mean that over-tightening a joint can cause serious damage to the thread of the fastener being tightened or to the component being anchored by the threaded fastener, whereas under-torquing a joint does and always has had serious safety issues.
On a typical production line, an assembly engineer may use a torque wrench that is pre-set to deliver a predetermined amount of torque before the wrench sends a haptic feedback signal to the user to warn that the correct torque level has been applied to the joint. The predetermined amount of torque applied by the wrench to a joint which triggers the haptic feedback signal is known as the set point of the wrench. The most common torque wrenches used in industry are so-called click wrenches. Each wrench comprises a handle and a working head connected together by a shaft. The length of the shaft determines how much torque is applied at the working head by the user imparting a given manual force to the handle. The working head may be a simple rigidly mounted square socket coupling or it may include a ratchet mechanism mounting a square socket coupling. The click mechanism creates the haptic feedback to the user when the desired set point has been reached, which feedback comprises a limited small angular movement between the shaft and the working head which is permitted only when the set point has been reached. The shaft and working head are locked at a constant angle when the applied torque is less than the set point, but when that set point is reached a trigger releases the locking and allows the above small angular movement, generally of only one or two degrees of angle, before again locking the shaft and the working head at a (second) fixed angle for example by abutment of a portion of the shaft against a fixed wall of the working head or vice versa. That sudden movement normally generates a click sound, from which the click wrench takes its name; and the click sound does provide the user with a degree of aural feedback indicating that the set point has been reached, although the much more discernible haptic feedback is the feel of sudden and abruptly terminated small free movement of the handle as the user applies a force to the wrench at the handle end. The user relies on that haptic feedback to tell him or her to cease applying force to the handle end of the wrench. Continued application of force will cause over-tightening of the joint, and the click wrench relies on the skill of the user to release the force on the handle as soon as the haptic feedback is sensed. Current working practices are such that a user may have access to a first click wrench pre-set to deliver a recommended torque of, for example, 40 Newton Meters (Nm) to a first range of joints; a second click wrench pre-set to deliver a recommended torque of, for example, 30 Nm to a second range of joints; and third and further click wrenches set to deliver different recommended torques to other joints on the assembly line. The potential disadvantages of this practice are immediately apparent. The user may pick up the wrong wrench to use on a given joint. Even if that does not happen, each user must be provided with a sufficient number of differently pre-set click wrenches to accommodate all of the joints being fastened; and each of those pre-set wrenches must be maintained at the correct torque setting and regularly calibrated to make sure that the set point does not wander from the intended setting in use. Recalibration of every single click wrench on a weekly basis is not uncommon. Some click wrenches are user-adjustable so that the user may alter the set point against a dial or scale provided on the wrench itself, so that the same click wrench may be used to tighten different joints to different desired torques. That has the advantage that a single click wrench can be used in place of several, but the disadvantage that it relies on the user to remember to reset the set point whenever moving from a joint with one desired torque level to another; and it relies on the user to make that adjustment accurately. As with the non-user-adjustable click wrenches, such adjustable wrenches need to be recalibrated and serviced regularly, to ensure that the set point at which the click mechanism is triggered is accurately reflected on the dial or scale.
A simple mechanical click wrench triggers the haptic feedback indicating that the desired set point has been reached by a trigger mechanism, generally a roller ball which is normally held in a concave seat by a spring, which is purely mechanical and which relies on the compression of the spring to control the desired set point. The spring compression must be checked regularly, to maintain accuracy of the haptic feedback signal.
All such simple mechanical click wrenches have the limitations that (a) they cannot record the actual torque to which a joint has been tightened and (b) they do not monitor the angular movement of the wrench head during tightening.
No simple click wrench can however provide a guarantee that the user has tightened any given joint to its recommended torque value. The user may not respond properly to the haptic feedback and may over-tighten or under-tighten any particular joint. Much greater reliability, and a record of the torques to which a series of joints have been tightened, is provided by electronic torque measurement of the joints being tightened, which is possible using a bending beam and a strain sensor or sensors on that bending beam with a feedback of measured maximum torque being relayed to a computer memory. That enables the computer to monitor the sequence of fasteners being tightened, and by incorporating a sensor which recognises each joint being tightened, to set the desired threshold torque electronically for each joint in turn in the sequence. It has been proposed to insert a separate strain sensor as an additional element in the torque application path between the working head of a mechanical wrench such as a click wrench and the socket which drives the head of the fastener being tightened. Such a separate strain sensor does however incur an additional cost and can be removed and mislaid by the user. It does not create automatic electronic adjustment of the desired set point for any given joint being tightened. The addition of a separate strain sensor between the wrench head and the joint adds to the overall length of the wrench. This has inherent disadvantages. In the first place users do in general prefer smaller and shorter wrenches, which provide better control of torque application and are less susceptible to over-torquing. In addition, the insertion of a separate strain sensor between the wrench head and the joint requires an operator to compensate for the additional torque which a given pulling force will exert at a joint. The user may need to have reference to a look-up table or may perform actual calculations to provide that compensation, and the calculations are in any case predicated on the user pulling the click wrench at a specific point on the handle.
It has also been proposed to incorporate such a strain sensor or sensors into the shaft of a torque wrench as a permanent feature, to display the applied torque on an electronic display on the wrench handle or shaft, and to generate a feedback signal to the user from the resulting electronic torque measurement when the set point is approached or reached. The electronic display is more accurate than the purely mechanical display of the dial or scale of the adjustable mechanical click wrenches discussed above. The most easily generated feedback signals are visual or aural. For example a light or a series of lights on the wrench or on a small monitor adjacent the user can indicate when the desired torque is approached and/or attained, or an audible alarm could sound to indicate the same. Such visual or aural feedback signals are however easily overlooked in a factory environment where there may be background noise and distracting lights, or when the wrench is used at an awkward angle or in a position where the visual display is difficult to see. There has therefore been a need for a mechanism to trigger a haptic feedback in response to an electronic torque measurement within the wrench, so that the benefits of an electronic torque wrench can be combined with the familiarity and ease of use of a click wrench. Although there have been proposals to combine electronic torque sensing and a click mechanism, for example in US-A-2007/0227316 or US-A-2011/0132157, no such electronic click wrench has been offered on the commercial market. The reason is apparently the difficulty of providing a sufficiently sensitive and reliable trigger that is responsive to relatively small trigger forces in a wrench which has a torque path designed to deliver torques much higher than the trigger torques, for example torques of up to several hundreds of Newton Meters. Commercially available electronic wrenches therefore still tend to use visual or aural feedback to the user.
It is an object of the invention to provide an electronic torque wrench which includes a haptic feedback of the click mechanism variety, while maintaining a reliable triggering of the haptic feedback when a desired set point has been sensed by strain sensors in the wrench.